This invention relates to pressure transducers, as microphones.
A known type of pressure transducer, the "Sell-transducer," comprises a base body having a rough or fluted surface, and a thin, flexible insulating diaphragm, as a plastics film, abutting said surface and bearing a thin metal layer on the side remote of said surface. An air cushion or space formed between said base body surface and the diaphragm allows the diaphragm to oscillate in response to pressure variations, as sound waves, and determines, with other parameters, the resonant frequency of the system. For detecting sound waves having frequencies in the higher ultrasonic range, a small roughness of the base surface is preferred. An electric output signal is generated, similar as in a condensor microphone, by detecting the capacity variations between the conducting surface of the base body and the outer metal layer of the diaphragm (Acustica 4 (1954), 519 to 532).
To operate the known microphone mentioned above, a high d.c. bias or polarization voltage is needed which usually amounts to several hundred volts, and which undesirably stresses the very thin diaphragm to its limit of electric breakdown. Further, the output signal is a relative small voltage originating from a source of relative high impedance. Thus, an impedance transformer has to be provided close to the microphone which is often a source of noise during measurements and which, for its part, needs an own voltage supply.
An object of the present invention is to provide a microphone of the above general type which does need no high polarization voltages and no impedance transformer.